tern on the photographic plate. By studying the angle and intensity of 

 diffraction produced by the various lattice planes of the crystals, one 

 can gain much information concerning their symmetry. 



Many biological materials (e.g., cellulose, keratin, collagen fibers, 

 hemoglobin) are made up of repeating units of similar molecules which 

 tend to show some form of preferred orientation with respect to the main 

 axis of the material. The angle at which the repeating units diffract 

 X rays of known wavelength is a measure of the distance between each 

 repeating unit, that is, the greater the angle of diffraction, the smaller 

 the distance between the repeating units. The preferred orientation of 

 the repeating molecules is revealed by the pattern of interference figures 

 forming the diffraction diagram. The X-ray diffraction method has been 

 a contributing factor in the construction of a statistical model of the 

 DNA molecule. One of the possible diffraction patterns which has been 

 obtained for DNA is shown in Figure 11-35. 



AUTORADIOGRAPHY 



Autoradiography involves placing cells or tissues which have been 

 previously exposed to a radioactive isotope in contact with a photo- 

 graphic emulsion (Figure 11-36). The ionizing radiations emitted by 

 the incorporated isotope cause a blackening of the emulsion and produce 

 an image of the specimen which can be used to determine the areas 

 containing radioactive material. 



In preparing specimens for autoradiography, it is common practice to 

 fix them before the photographic emulsion is applied. The method of 

 fixation depends on the chemical constituents to be studied. For the 

 study of radioisotope incorporation into the nucleoproteins of cell struc- 

 tures such as the chromosomes, Carnoy's acetic-alcohol is generally 

 employed. When it is desirable to retain all of the incorporated radio- 

 active material, tissues may be quick-frozen and dried by vacuum desic- 

 cation. In order to follow the incorporation of an isotope into a specific 

 chemical constituent, such as RNA, it is necessary that all DNA be 

 extracted from the tissue. If incorporation of the isotope in DNA is to 

 be investigated, all the RNA must be extracted. The differential extrac- 

 tion of proteins is more difficult, since adequate methods of extraction 

 are not available for the separation of all the different kinds of proteins. 

 Basic proteins (histones) apparently can be extracted sufficiently to per- 

 mit comparison of isotope incorporation into basic versus nonbasic 

 proteins. 



SURVEY OF CYTOLOGICAL TECHNIQUES / 257 



